Method and system for feedback control of optical fiber lens fusing

ABSTRACT

A method and system for fusing an optical fiber lens is compatible with automation. Specifically, the fusing of the fiber lens is controlled in response to a diffraction pattern of light exiting from the fiber lens. This diffraction pattern is indicative of the lens shape and characteristics. Specifically, light is injected into an optical fiber and a diffraction pattern of the light exiting from a fiber lens is detected. The fiber lens is then fused in response to this diffraction pattern.

RELATED APPLICATION

[0001] This application is a Continuation of U.S. application ser. No.09/740,430, filed on Dec. 19, 2000, entitled “Mghod and. System forFeedback Control of Optical Fiber Lens Fusing,” by Flanders et al, nowabandoned. The entire teachings of which are incorporated herein by thisreference.

BACKGROUND OF THE INVENTION

[0002] In fiber optic systems, optimization of coupling efficiencybetween the optical fiber via the fiber's end face and active and/orpassive devices is an important metric for comparison at the systemlevel. A popular technique for improving coupling efficiency includesproviding a lens at the fiber end face. This lens can be formed bydrawing or etching the fiber. Generally, however, fiber-polishingtechniques yield the best and most consistent fiber lens profiles.

[0003] Fiber polishing, however, often yields sharp surface features ormicroscopically rough surfaces. To smooth these features and round-oversharp corners, it is common to expose the polished fiber lens to afusing step.

SUMMARY OF THE INVENTION

[0004] The typical approach to fusing is to carefully monitor thepositioning of the fiber end face between the fuser electrodes incombination with optimizing the fuser current and fusing duration. Asimilar approach can be used with flame fusing. After repeating manyexperiments, an experienced technician can generate fiber lenses withgood profiles at acceptable yields. It is further common to includevisual inspection techniques between fuser exposures to monitor theprogress of the fusing operation.

[0005] The present invention concerns a method and system for fusing anoptical fiber lens. The system is compatible with automation.Specifically, the fusing of the fiber lens is controlled in response toa diffraction pattern of light exiting from the fiber lens. Thisdiffraction pattern is indicative of the lens shape and characteristics.Further, it is generally is easier to assess the lens shape from thediffraction pattern rather than visual inspection.

[0006] In general, according to one aspect, the invention features amethod for fusing an optical fiber lens. This method comprises injectinglight into an optical fiber and detecting a diffraction pattern of thelight exiting from a fiber lens at a proximal end of the optical fiber.The fiber lens is then fused in response to this diffraction pattern.

[0007] In the preferred embodiment, the step of injecting light into theoptical fiber comprises energizing a laser that is coupled to a distalend of the optical fiber. In an alternative embodiment, a techniquesimilar to that used in fiber coupling may be used where the fiber isbent and light injected into the core through the cladding.

[0008] According to other aspects of the preferred embodiment, the stepof detecting the diffraction pattern comprises detecting a far-fielddiffraction pattern. This is preferably performed using atwo-dimensional detector, such as a CCD camera detector that is locatedoptically in front of the fiber lens—the detector need not be locatedphysically in front of the fiber lens if there is intervening foldoptics, for example.

[0009] In one present embodiment, the diffraction pattern is analyzed bydetermining a ratio of a lateral size to a transverse size of thediffraction pattern. The fiber lens is exposed to a fusing arc until theratio of the transverse to lateral size reaches a desired ratio.

[0010] In general, according to another aspect, the invention can alsobe characterized as a system for fusing an optical fiber lens. Thissystem comprises a light source that injects light into an optical fiberand a detector that detects a diffraction pattern of a light exitingfrom a fiber lens at a proximal end of the optical fiber. An arc fuseris disposed to fuse this fiber lens. A controller activates the fuser inresponse to the diffraction pattern detected by the detector.

[0011] The above and other features of the invention including variousnovel details of construction and combinations of parts, and otheradvantages, will now be more particularly described with reference tothe accompanying drawings and pointed out in the claims. It will beunderstood that the particular system and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In the accompanying drawings, reference characters refer to thesame parts throughout the different views. The drawings are notnecessarily to scale; emphasis has instead been placed upon illustratingthe principles of the invention. Of the drawings:

[0013]FIG. 1 is a schematic, block diagram of an optical fiber lenselectrofusing system according to the present invention;

[0014]FIG. 2 is an image of a polished fiber lens prior to fusing;

[0015]FIG. 3 is a far field diffraction pattern of light exiting from afiber lens prior to beginning the fusing operation;

[0016]FIGS. 4A through E show a far field diffraction pattern after 500,2500, 6500, 9000, and 11000 milliseconds of fusing, respectively,according to the present invention; and

[0017]FIG. 5 is an image of a fiber lens after the fusing operation hasbeen completed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018]FIG. 1 shows an optical fiber lens electrofusing system, which hasbeen constructed according to the principles of the present invention.

[0019] Specifically, a laser source 110 injects light into a distal end112 of an optical fiber 114. In the current embodiment, the fiber issingle mode fiber. In alternative embodiments, polarization-maintainfiber is used. In alternative embodiments, a light source can be usedthat injects light through a cladding of the optical fiber 14 afterbending the fiber, for example.

[0020] The light 116 is emitted from an end face of the fiber 114. Inthe current implementation, an intermediary fiber lens 118 has beenformed on the end face of the optical fiber 114 by a polishing process.

[0021] The light 116 exiting from the lens 118 forms a diffractionpattern. This diffraction pattern is detected by a camera or detector121. In the preferred embodiment, this is a CCD detector that has a twodimensional array of detecting elements as are found in conventionalcamera-type detectors.

[0022] In the preferred embodiment, the detector 121 is positionedrelative to the lens 118 to detect a far-field diffraction pattern. Thefar-field region is typically defined as the region where the angularfield distribution is essentially independent of distance from thesource. If the source has a maximum overall dimension D that is largecompared to the wavelength, the far-field region is commonly taken toexist at distances greater than 2D²/λ from the source, λ being thewavelength, which is typically between 1300 to 1600 nanometers. In thecase of single mode fiber, D is between 6 and 10 micrometers, typically.

[0023] The camera 121 is connected to a controller 122. This controllerdetects or analyzes the diffraction pattern detected by the camera. Inone implementation, the controller compares a width or lateral size ofthe diffraction pattern formed by light 116 on the camera 121 to aheight or transverse size of the light diffraction pattern formed on acamera 121. Based on this information, the controller compares thedetected diffraction pattern and specifically the derived parameters tooptimal far field parameters stored in an optimal far field parameterstorage 122.

[0024] Based on the comparison of the detected far field and the optimalfar field pattern, the controller 120 operates a fuser system comprisinga fuser driver 124, which is typically a high voltage source, thatdrives a current 126 between electrodes 128, 130. The fiber lens 118 isphysically placed between the electrodes 128, 130, so that it is exposedto the resulting plasma, and thereby heated, resulting in the fusing theend face 118.

[0025] According to the invention, an electrofuser system is used. Thesehave advantages relative to flame and laser fusing systems.Specifically, electrofuser typically have lower placement tolerances ofthe fiber between the electrodes in order to obtain reproducibleresults.

[0026]FIG. 2 is an image of an intermediate fiber lens 118 immediatelyafter the polishing operation and prior to fusing. As can be seen, thereare striations on the surface of the lens that are artifacts of thepolishing process. Moreover, it has relatively sharp corners at, forexample, the tip 118A.

[0027] When this fiber lens is installed into the fusing system, itproduces a far field diffraction pattern as shown in FIG. 3. Thecontroller 120 recognizes this far field diffraction pattern as beingsuboptimal and as a result, begins the fusing operation.

[0028]FIG. 4A shows the far field diffraction pattern after 500milliseconds (msec) of fusing. One embodiment, the 500 msec of fusing isperformed in a single long pulse. In another embodiment, the 500 msec offusing is a concatenation of 100 msec pulses.

[0029] The controller 120 calculates a lateral to transverse size of thepattern. In one implementation, this ratio is calculated based upon a 3dB width (full width, half maximum) of the maximum pixel intensitycontour. This ratio is 6.11 for FIG. 1. In one embodiment, the optimalratio stored in storage 122 is 2.75.

[0030] Thus, the end face is fused for another 2000 msec and the farfield pattern is detected as shown in FIG. 4B. The resulting ratio is4.55. The end face is fused for another 4000 msec and the pattern ofFIG. 4C is detected. There the ratio is 3.45, which is still not withinan acceptable window around the optimal 2.75 ratio. Two more successivefusings of 2500 msec and 2000 msec are performed, see FIG. 4D and 4E,having ratios 3.12 and 2.74, before the optimal ratio is achieved

[0031]FIG. 5 shows the resulting end face after the fusing operation hasbeen performed. The fiber lens 118 now has a much smoother profile.Moreover, the surface striations associated with the polling operationhave been removed.

[0032] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method for fusing an optical fiber lens, comprising: injectinglight into an optical fiber having a wedge-shaped fiber lens formed bypolishing at a proximal end of the optical fiber; detecting adiffraction pattern of the light exiting from the fiber lens; andelectro-fusing the fiber lens in response to a two-dimensionaldistribution of the diffraction pattern; and analyzing a two-dimensionaldistribution of the diffraction pattern by determining a ratio of alateral size to a transverse size of the diffraction pattern.
 2. Amethod as claimed in claim 1, wherein the step of injecting the lightinto the optical fiber comprises energizing a laser that is coupled to adistal end of the optical fiber.
 3. A method as claimed in claim 1,wherein the step of detecting the diffraction pattern comprisesdetecting a far-field diffraction pattern.
 4. A method as claimed inclaim 1, wherein the step of detecting the diffraction pattern comprisespositioning a two-dimensional detector optically in front of the fiberlens.
 5. (cancelled)
 6. (cancelled)
 7. A method as claimed in claim 1,wherein the step of fusing the fiber lens comprises exposing the fiberlens to an electrical arc.
 8. A system for fusing an optical fiber lens,comprising: a light source that injects light into an optical fiber; adetector that detects a two-dimensional distribution of a diffractionpattern of the light exiting from a fiber lens at a proximal end of theoptical fiber, the fiber lens being wedge-shaped and having been formedby polishing; an arc fuser that fuses the fiber lens; and a controllerthat activates the arc fuser in response to the two-dimensionaldistribution of the diffraction pattern detected by the detector anddetermines a ratio of a lateral size to a transverse size of thediffraction pattern.
 9. A system as claimed in claim 8, wherein thelight source comprises a laser that is coupled to a distal end of theoptical fiber.
 10. A system as claimed in claim 8, wherein the detectoris positioned relative to the fiber lens to detect a far-fielddiffraction pattern.
 11. A system as claimed in claim 8, wherein thedetector is positioned greater than 0.5 centimeters from the fiber lens.12. A system as claimed in claim 8, wherein detector comprises a camera.13. (cancelled)
 14. (cancelled)
 15. A system as claimed in claim 8,wherein the controller activates the arc fuser in a pulsed fashion untila desired diffraction pattern is detected by the detector.
 16. A methodfor fusing an optical fiber lens, comprising: injecting light into anoptical fiber having a wedge-shaped fiber lens formed by polishing at aproximal end of the optical fiber; detecting an aspect ratio of adiffraction pattern of the light exiting from the fiber lens bypositioning a two-dimensional detector optically in front of the fiberlens; and electro-fusing the fiber lens in response to the aspect ratioof the diffraction pattern by exposing the fiber lens to an electricalarc until an optimal aspect ratio is detected.
 17. A method as claimedin claim 16, wherein the step of electro-fusing the fiber lens byexposing the fiber lens to the electrical arc comprises exposing thefiber lens to electrical arc pulses.